ABSTRACT:

This paper presents a new efficient rock breaking method of Coiled Tubing Partial Underbalanced Drilling (CT-PUBD). Unlike conventional underbalanced drilling, it can achieve a partial underbalanced condition near the bit rather than the whole borehole. The rate of penetration is influenced by hole cleaning. In this technique, most cuttings are translated in micro annulus which is between drill pipe and coiled tubing. Moreover, the micro annulus where the drill pipe rotates and coiled tubing is static is different from normal annulus which is between borehole and drill pipe. In this paper, High Speed Photogrammetry System is used to observe the cuttings transportation. It can be found that the cuttings move as spiral movement in the micro annulus and overturn continuously. With the increase of rotational speed, the cuttings tangential velocity increases. As the displacement increases, the cuttings axial velocity increases. The relationship between the cuttings velocity and the drilling fluid velocity is established by defining cuttings tangential velocity coefficient and cuttings axial velocity coefficient. The cuttings tangential velocity coefficient and cuttings axial velocity coefficient range from 0.83 to 0.88 and 0.29 to 0.46 respectively.

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